The present invention relates to a martensitic, corrosion resistant steel for rock drilling, with new and improved properties, particularly with regard to resistance against corrosion fatigue.
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
During percussive rock drilling shock waves and rotation are transferred from a drill machine via one or more rods or tubes, to a cemented carbide equipped drill bit. The drill steel, i.e. the material in bits, rods, tubes, sleeves and shank adapters is subjected to corrosive attack. This applies in particular to underground drilling where water is used as flushing medium and where the environment in general is humid. The corrosive attacks are particularly serious in the most stressed parts, i.e. thread bottoms and thread clearances. In combination with pulsating stress, caused by bending stresses and the above-mentioned shock waves, so-called corrosion fatigue arises (FIG. 1). This is a common cause of failure of the drilling steel.
A low-alloyed, case hardened steel is normally used for the drilling application. The reason is that abrasion and wear of the thread parts have generally been limiting for life. As the drilling machines and tools have become more efficient, these problems have diminished and corrosion fatigue has become a limiting factor. The case hardening gives compressive stresses in the surface, which gives certain retarding effects on the fatigue.
U.S. Pat. No. 5,496,421 relates to a high strength martensitic stainless steel. The steel contains: 0.06 wt-% or less C, 12 to 16 wt-% Cr, 1 wt-% or less Si, 2 wt-% or less Mn, 0.5 to 8 wt-% Ni, 0.1 to 2.5 wt-% Mo, 0.3 to 4 wt-% Cu, 0.05 wt-% or less N, and the balance being Fe and inevitable impurities; said steel having an area ratio of delta-ferrite phase of at most 10%. The known steel intends to solve the problem of stress corrosion caused by an acidic environment.
One object of the present invention is to provide an elongated element for percussive rock drilling which further improves the efficiency of modern mining.
Another object of the present invention is to provide an elongated element for percussive rock drilling with increased life.
Still another object of the present invention is to provide a drill steel with reduced corrosion rate.
Still another object of the present invention is to provide a drill steel with reduced sensitivity for corrosion fatigue.
According to one aspect, the present invention provides a steel for an elongated element used in percussive rock drilling including at least a thread and flush channel, the steel is corrosion resistant and has a mainly martensitic microstructure.
According to a further aspect, the present invention provides a steel adapted for use in percussive rock drilling, the steel being corrosion resistant and having a martensite content of  greater than 50 wt-% but  less than 100 wt-%, the steel having a composition comprising at least one of:
C+Nxe2x89xa70.1 wt-% and Crxe2x89xa711 wt-%;
C+Nxe2x89xa70.1 wt-% and Crxe2x89xa75 wt-%, Moxe2x89xa65 wt-%, Wxe2x89xa65 wt-%, Cuxe2x89xa62 wt-%;
Mo+W+Cu greater than 0.5 wt-%; or
C+Nxe2x89xa70.1 wt-% and Cr+3.3(Mo+W)+16N greater than 10 wt-%.
According to one aspect, the present invention provides an elongated element for percussive rock drilling including at least a thread portion and flush channel, at least the thread is made of a corrosion resistant steel with a mainly martensitic structure.
According to another aspect, the present invention provides an elongated element for percussive rock drilling including at least a thread portion and flush channel, at least the thread portion is made of a corrosion resistant steel having a martensite content of  greater than 50 wt-% but  less than 100 wt-% and that the steel has a composition comprising at least one of:
C+Nxe2x89xa70.1 wt-% and Crxe2x89xa711 wt-%;
C+Nxe2x89xa70.1 wt-% and Crxe2x89xa75 wt-%, Moxe2x89xa65 wt-%, Wxe2x89xa65 wt-%, Cuxe2x89xa62 wt-%;
Mo+W+Cu greater than 0.5 wt-%; or
C+Nxe2x89xa70.1 wt-% and Cr+3.3(Mo+W)+16N greater than 10 wt-%.
According to yet another aspect, the present invention provides a use for a steel having a mainly martensitic microstructure, the use comprising forming at least a thread portion of an elongated element for percussive rock drilling from the steel.